The industrial action, which starts today, is being taken by members of the GMB union, with further strikes planned for 5 November and 9 November.

GMB regional organiser David Warwick said the senior management of NASUWT had not been willing to engage in “meaningful discussions” to resolve the dispute over changes to the pension scheme and a three-year pay deal that were “both imposed without the agreement of our members”.

Under NASUWT proposals, staff will have to work for 45 years instead of 40 to fund a half-salary pension. GMB insisted the pay deal imposed by NASUWT, representing a rise equivalent to RPI inflation, currently at 3.1%, “effectively removed members’ ability to renegotiate a reopener clause that was triggered in the previous two-year pay deal because inflation had outstripped it”.

The industrial action follows a ballot in which 87.6% of GMB members voted to strike, from a turnout out of 77.6%.

Warwick said: “As a last resort, GMB members are now taking industrial action to prevent cuts to their pension scheme and to protect their rights to collective bargaining. We are hoping this action will make the employer recognise the strength of feeling of their staff as to what is happening and do something about it.”

NASUWT offices around the country are affected, with staff striking at the headquarters in Rednal in Birmingham, as well as London, Scotland, Bury St Edmunds, East Midlands, Exeter, Newcastle, Surrey and Yorkshire.

A NASUWT spokesman said employees had been guaranteed a “defined benefits pensions scheme” and a pay award equivalent to RPI over the next three years, and that it was “disappointing” that the GMB had rejected its offer.

He added: “The NASUWT apologises for any inconvenience the GMB’s action may cause. We will make every effort to minimise any disruption caused by the GMB strike action, to keep NASUWT sites open and to maintain its service to members.”